GAP JUNCTIONAL COMMUNICATION AND PHARMACOLOGICAL HETEROGENEITY IN ASTROCYTES CULTURED FROM THE RAT STRIATUM

1. Indo-1 and fluo-3 imaging techniques were used to investigate the r ole of gap junctions in the changes in cytosolic calcium concentration s ([Ca2+],) induced by several receptor agonists. Subpopulations of co nfluent cultured astrocytes from the rat striatum were superfused with submaximal concentrations of endothelin-1 (Et1) and the alpha 1-adren ergic and muscarinic receptor agonists, methoxamine and carbachol, res pectively. 2. Combined binding and autoradiographic studies indicated that all striatal astrocytes possess binding sites for Et1. In contras t, alpha 1-adrenergic and muscarinic binding sites were found to be he terogeneously distributed. In agreement with these findings, Et1 induc ed fast calcium responses in all cells while only subsets of striatal astrocytes responded to the application of either methoxamine or carba chol. 3. Halothane, heptanol and octanol, which are commonly used as g ap junction inhibitors, drastically reduced the amplitude of Et1-induc ed calcium responses. In contrast, 18-alpha-glycyrrhetinic acid (alpha GA) used at a concentration known to block gap junction permeability in astrocytes had no significant effect on the amplitude of these calc ium responses. 4. As demonstrated by quantitative and topological anal ysis, Et1 application similarly increased [Ca2+](i) levels in all astr ocytes in both the absence and presence of alpha GA. 5. In control con ditions, subpopulations of cells responding to methoxamine or carbacho l exhibited two main types of calcium responses which differ ed in the ir shape and kinetic characteristics. In the presence of alpha GA the number of cells responding to these receptor agonists was significantl y reduced. Indeed, responses characterized by their long latency, slow rise time and weak amplitude disappeared in the presence of alpha GA while responses with short latency and fast rise time were preserved. 6. These results indicate that permeable gap junction channels tend to attenuate the pharmacological and functional heterogeneity of populat ions of astrocytes, while their inhibition restricts calcium responses in astrocytes expressing high densities of transmitter receptors coup led to phospholipase C.